Go use those Traits

- (10 min read)

Interfaces were popularized by Java but originally came from Objective-C (source).Until a few days ago I did not realize how powerful they were. In my opinion, interfaces have been potrayed in an incomplete manner by all the tutorials. The key idea is correct but it limits the thinking of what it can be used for.

I am sure everyone who has written some form of object oriented language would know this example.

public interface Animal {
    public String speak();

public class Cat implements Animal {
    public String speak() { return "I am a God !"; }

public class Dog implements Animal {
    public String speak() { return "Where's my hooman !"; }

And this being used as follows:

Animal[] animals = new Animal[2];
animals[0] = new Cat();
animals[1] = new Dog();

for (Animal a : animals) {

This shows a very generic and boring use of interfaces. Accepting multiple types to make a function generic or grouping a certain group of objects by an interface type. You would have seen things like this.

public interface Animal {}

public class Cat implements Animal {
    public void walk() { /* Some funny walk */ }
    public void beWeird()  { /* Need I say anything */ }

public class Dog implements Animal {
    public void beGoodDoggo() { /* Woof ! */ }
    public void bathe() { throws new RuntimeException(); }

These 2 classes have no common functionality, yet they share an interface. This is just to mark these classes in a common group, logically. Useful and bit interesting use too, but we have something better.

How do you test a function that takes a filepath as a parameter

I am sure you would have written, at some point of time, a function that takes a file path as an input and you need to do certain operations on it, maybe read to it, or write to it and return. Something like this.

public int findCharInFile(String path, byte c) {
    * Open the file by creating a Reader over an input stream and then read
    * operations.
    return Integer.MAX_VALUE;

How would you write tests for it ? You could do a dummy test file in your project with the test strings. For a long time I thought that was an ok way. Recently, I realized, there are better ways to implement it with higher testability.

What's problematic with that interface design

The above API isn't wrong, but it has some problems with it.

  • Testing becomes a problem, if you have more of those functions, your project will have a lot of stray files which are filled with different test cases.

  • The function signature doesn't convey it needs a file and not some random string. Arguably, it would be just made as findCharInFile(File file, byte c) but it's a generalized perspective with all the languages.

  • You don't know anything about the ownership of the file, what should you do once you get that File object.

  • And the most fascinating, what do you do when it's not a regular file.

How to solve the above problems

We write a function signature that expresses what is intended only. Every construct in a language has constraints on it. The less the constraints the more flexible it becomes. The real intent here is to read some bytes from a stream and then do some processing over it. Why limit it to a file ?

Enters the abstract class, java.io.Reader. It defines exactly the behaviour we need. An Entity, we don't care what, with a read function.

public int findCharInFile(Reader rd, byte c) throws IOException {
    char buf[] = new char[4096];
    int pos = 0;
    while (rd.read(buf) != -1) {
        /* Find the char by interating the buffer. */

    return pos;

Since, the java.io.Reader implements java.io.Closeable iterface as well, it gives an API for the caller to be able to close the stream. This might give a bit more control by default as there is no way to not allow the caller to close the stream without wrapping it into another class that does not expose those APIs.

How do you test it ?

public class DummReader implements Reader {
    /* implement all the methods required by the Reader abstract class */

and you can pass this to the function.

Let's look at some newer langauges and in some more detail.

golang is the new language for the writing webservers and things on the internet. I am looking at you, NodeJS.

I feel bringing JS to the backend was a mistake, or atleast not regulating it. I have an imperative programming background. Also, I have worked a bit on a few NodeJS projects. For some reason, it didn't seem receptive to me and it had certain inconsistencies that made it hard for me to write correct code. I know a lot of the veterans (irrespective of their backgrounds) will disagree with me, but I didn't feel that with other languages, so, just my opinion, no one needs to agree to it.

Back to go, it's an incredibly well designed language with constructs that are simple to use and most importantly, easy to read. It looks minimalist yet has a loaded standard library.

Go has 2 interfaces that has the power to change the way you write code. They are:

There are other interfaces too that can help you write even cleaner and self explanatory code, but let's start with this.

Let's see how the same function would look in go, but with these constructs.

func findCharInFile(rd io.Reader, c rune) int {
    contents, _ := ioutil.ReadAll(rd)
    return strings.IndexRune(contents, c)

This is piece of code has acheived the following things,

  • The function signature clarifies exactly what it needs. The io.Reader interface requires exactly one implementation, that is Read([]byte) (int, error). This function has no more bussiness than reading the file.

  • This is much more unit-testable. All you need is a construct that implements the io.Reader interface. You can mock one up and send it to the function or use one of the builtins. eg:

    func TestFindCharInFile(t *testing.T) {
      // Creates a *Reader type object which uses the supplied string as it's
      // backend and is Read only.
      rd := strings.NewReader("Contents of your test string")
      if findCharInFile(rd, rune('$')) != -1 {
  • This expresses the permissions you have on the stream. The io.Reader only has a Read([]byte) function, so you can't use it to do anything else with it unlike when you have the file path and you are responsible for opening and closing it.

  • It abstracts the underlying provider of the readable stream. It could be a file on the disk, an in-memory buffer, some key value store, even the network. It just has to adhere to one property or should have atleast the Read trait, to be able to Read out bytes to a bytes buffer.

  • It also tells you the ownership of the underlying object. The caller is only only allowed to read the stream and not do anything else with it, like close the stream when it thinks it's done. Maybe other's are still reading it, that would cause a panic, quite literally.

The same thing goes with io.Writer. It's only allowed to Write([]byte) (int, error) and will not allow the user to close it.

What if you wanted the user to close it.

If you want to hand over or delegate the control of the stream to the caller, you can use one of the derived interfaces, ReadCloser.

type ReadCloser interface {

You can hand this out and let the caller be able to close the stream. There are other such interfaces that Go provides by default and you can create your own too.

Can Rust do it

We can't not talk about Rust in this case. The title mentions to use the trait related. which is the Rust way of definiting features. Every trait can be seen as an interface and whosoever implements the trait can be said to have implemented that interface.

How would the same code (thanks to vlisivka) look in Rust.

fn find_char_in_file(rd: &mut dyn Read, c: u8) -> Result<Option<usize>> {
    for idx, b in rd.bytes().enumerate() {
        if b? == c {

Notice the definition,

find_char_in_file(rd: &mut dyn Read, c:u8)

This conveys that the receiver rd must implement the io::Read trait and that's all is required for this function to work. Similarly it's easy to test too.

mod tests {
    use super::*;

    struct TestReader();

    impl Read for TestReader {
        fn read(&mut self, &mut &[u8]) -> Result<usize> {
            // Ignoring all the implementation details.

    fn test_find_char_in_file() {
        let mut rd = TestReader::default();
        let res = find_char_in_file(&mut rd, 'c' as u8).unwrap();
        assert_eq!(res, None);

What about buffering, I don't want to do that on my own.

The io::Read trait takes care of that. The buffered Reads are auto implemented traits given the read() implementation. So, all you need is to write a simple read and rust will take care of providing a more efficient Buffered read.

The same is possible in go, using the bufio package. If you want to imply to the caller that you need a more efficient read implementation in your interface, you can simply replace the io.Reader with bufio.Reader interface which implements buffering over an io.Reader for you.

So, it would become something like,

func findCharInFile(rd bufio.Reader, c rune) int

Converting a normal unbuffered reader to a buffered one is easy,

bufRd := bufio.NewReader(rd)

What did I learn

We haven't even talked about doing the same operations with an underlying network connection and not a file or a device or an in-memory buffer. But it would work just fine as network connections too are just Read and Write calls.

This helps simplify writing the code. Notice that those functions now no longer have to deal with opening and closing files, handling errors that are related to files.

This explicitly conveys what the caller is responsible for and what can he do with it, given that the API can do just what the interface allows.

Helps a ton with testability of the code, one of the "ilities" in software architecture.

Don't get me wrong, It's not that all of it can't be done in Java. Java also has interfaces around InputStreams and BufferedInputStreams, Readers and BufferedReaders etc. All of this can be accomplished in a similar idiomatic and clean way. But's it's too much to write, Java is too verbose ;)

It's just about a more useful outlook to interfaces.

Where did I learn this from

Obviously, I didn't come up with all this. I have been working on a go project and in order to write better and well organized code, I started looking around for best practices and found some amazing talks about it. I will link them below if someone wants to take a look (highly recommended).

Discussion thread here